An evaporated fuel purge system includes, for example, a fuel tank of an internal combustion engine, a canister, and a purge control valve. This evaporated fuel purge is so designed that evaporated fuel produced in the fuel tank is temporarily adsorbed to the canister. The evaporated fuel adsorbed to the canister is taken, together with fresh air introduced through the fresh air inlet in the canister, into the air intake system of the internal combustion engine through the purge control valve. However, when a crack or the like exists in a pipe or container constituting an evaporated fuel recovery path running from the fuel tank to the purge control valve through the canister, the evaporated fuel leaks to the outside and the effect of preventing the emission of evaporated fuel cannot be sufficiently attained.
Recently, strict leak check has become mandatory against evaporated fuel emissions from fuel reservoir systems, such as the fuel tank of the vehicle, into the atmosphere. For this reason, a variety of leak check systems for diagnosing leakage from the evaporated fuel purge system are proposed.
According to U.S. Pat. No. 5,890,474 (JP-A-10-90107: Patent Document 1), a module is placed on the atmosphere port side of a canister. In this module, a switchover valve for switching flow paths and a motor pump are connected and integrated with each other. A reference leak is caused by the motor pump with pressure as a result of changing the flow path by the switchover valve. Then, the state of leakage from an evaporated fuel recovery path is compared with the reference leak. More specifically, pressure is alternately applied by the motor pump, for example, to a reference orifice and to the atmosphere port side of the canister, that is, the evaporated fuel recovery path. The reference orifice is for providing leakage reference values established by the California Air Resources Board (CARB) and the Environmental Protection Agency (EPA). At this time, the voltage of the motor pump is measured in the respective cases, and the comparison is made by operation characteristic values, such as current consumption, obtained therefrom.
According to the prior art disclosed in JP-A-11-336619 (Patent Document 2), a detecting device for detecting the state of use of an air conditioner is provided for prevention of erroneous determination due to the influences of the vapor pressure of fuel. A determination value for reference leak is corrected according to the results of detection by the detecting device. When the air conditioner is in operation, the outdoor temperature is estimated to be high, and the fuel temperature is also considered to be high.
According to the prior art disclosed in JP-A-2000-205056 (Patent Document 3), the driving voltage for a motor pump is changed to shorten time required for diagnosing a leak. Immediately after a start of driving, the motor pump is driven on relatively high voltage to increase the amount of discharging from the motor pump. Thereafter, the voltage is returned to normal voltage to return the amount of discharging to the reference amount of discharging for leak diagnosis.
The above prior arts are not satisfactory. When the supply voltage of a battery or the like for driving a motor pump fluctuates, the driving voltage proportionally fluctuates, which varies the performance of the motor pump itself. For example, when the supply voltage has dropped due to deterioration in a battery, the driving voltage of the electric motor unit constituting the motor pump drops. As a result, the capability of the motor pump to apply pressure is lowered. This decrease in motor pump power takes place not only in pressure pumps which discharge air to apply pressure but also in vacuum pumps which suck air or the like to reduce pressure.
The reference pressure based on a reference orifice and the internal pressure in an evaporated fuel recovery path can be measured using a vacuum pump and compared with each other. The influences of some factors on the accuracy of comparative determination in this case will be described below.
FIG. 11A is a graph plotting pressure change characteristics with low supply voltage, and FIG. 11B is a graph plotting pressure change characteristics with high supply voltage. In these graphs of pressure change characteristics, the horizontal axes show elapsed time and the vertical axes show absolute pressure P. The elapsed time can be divided into, for example, four sections, section A to section E, in correspondence with the process of leak check. The reference pressure Pr and the internal pressure in evaporated fuel recovery path are evaluated in sections C and D, respectively. With the lowered supply voltage, illustrated in FIG. 11A, the performance of the vacuum pump degrades.
Accordingly, the reference pressure Pr approaches the atmospheric pressure Patm, and the magnitude of the negative pressure of reference pressure is also reduced (section C). Thus, the difference between the reference pressure Pr obtained by the reference orifice and the atmospheric pressure Patm is reduced. Therefore, the differences are reduced between three different pressure change characteristics: pressure change characteristics with φ0.5 mm which is the same as the size of the hole in the reference orifice; pressure change characteristics with φ more than 0.5 mm with which a large leak takes place; and pressure change characteristics without leak. As a result, the accuracy of leak detection for determining in which state of leakage the size of a leaking hole determined from internal pressure change in section D is can be impaired.
With the high supply voltage, as illustrated in FIG. 11B, the reference pressure Pr deviates from the atmospheric pressure Patm, and the magnitude of the negative pressure of the reference pressure Pr is increased (Section C). As a result, the difference between the reference pressure Pr and the atmospheric pressure Patm is increased. Therefore, it is likely that a relief valve for failsafe is opened before a desired reference pressure is reached. Once the relief valve is opened, a leak will not be detected. When the setting of valve opening pressure for the relief valve is increased, the pump power is excessively increased and the fuel tank is overloaded. Therefore, the rigidity of the fuel tank must be enhanced to ensure the sufficient strength of the fuel tank.
For the above reasons, it is difficult to enhance the accuracy of leak detection with the above prior arts. Therefore, there is a possibility that the leakage reference values established by CARB and EPA or stricter leakage reference values in the future cannot be met.
To avoid such a situation, a constant-voltage circuit is provided at a position between the battery and the motor pomp, the constant-voltage circuit can receive a input voltage which is larger than a predetermined value in a range of varying battery voltage. The battery voltage varies due to the deterioration of the battery. The practical battery voltage varies from 8 volts to 16 volts when a nominal voltage value is 12 volts for automobile. When the difference between the set value of the constant-voltage circuit and the practical battery voltage is larger than a certain value, the redundant electric energy corresponding to the voltage difference becomes a heat energy of the constant-voltage circuit. The heat generated by the constant-voltage circuit likely affects performance of electric devices such as an alternator and sensors. When a switchover valve and the motor pomp are assembled integrally, an output characteristic of the motor pomp is likely varied due to the heat generated by the circuit during leak check. The evaluating duration (section C) of the reference pressure and evaluating duration (section D) of evaporated fuel circulation are different from each other. Even if a leak hole and reference orifice have the same diameter, for example, φ0.5 mm, a deviation is caused between the reference pressure and a saturation pressure, thus the accuracy of leak inspection deteriorates.